Wavelength division multiplexing technologies, where multiple wavelengths are simultaneously transmitted for achieving high-speed and large-capacity communications, have rapidly developed with the progress in optical communication technology.
In the construction or experiment of a wavelength division multiplexing transmission system, multiple light sources with different wavelengths are necessary corresponding to the number of transmission channels. In general, wavelength selection in wavelength division multiplexing transmission cannot be freely conducted since there are some limitations such as to avoid a combination of wavelengths troubling the transmission, e.g., mixing of four light waves. Namely, an arbitrariness, accuracy and sureness in wavelength selection are desired. In general, such a light source is difficult to fabricate and is costly due to the lowered production yield. Further, one light source is needed for one wavelength, thus, in case of eight-channel wavelength division multiplexing transmission, eight light sources are necessary.
On the other hand, when lights from multiple light sources are transmitted through one transmission line, a wavelength division multiplexing device called coupler or multiplexer is necessary. As the wavelength division multiplexing device, for example, a N.times.1 coupler of fiber fusion type, a N.times.1 coupler of waveguide type, or AWG (arrayed waveguide grating) is used. In case of eight channels, for example, the insertion loss for a 8.times.1 coupler is about 10 to 12 dB and that for AWG is about 6 to 9 dB. Thus, the insertion loss in coupling is so big.
In order to provide arbitrarily different light source wavelengths, it is necessary to fabricate separately laser diodes (LDs) for light source corresponding to the necessary wavelengths. Therefore, the manufacturing cost must be increased. Further, the production yield is lowered due to the accuracy and sureness needed for providing desired wavelengths. Also, the insertion loss in coupling must be so big.
For example, in a conventional N.times.1 coupler composed of several 2.times.1 couplers, a 2.times.1 coupler theoretically causes an insertion loss of 3 dB at minimum. Namely, a 8.times.1 coupler causes an insertion loss of 9 dB theoretically. In fact, there occurs an insertion loss of about 10 to 12 dB.
Meanwhile, the insertion loss can be reduced by using a wavelength coupler called AWG. For example, in case of AWG with eight channels, the insertion loss is about 6 to 9 dB, which is smaller than that of the 8.times.1 coupler. However, the insertion loss in coupling is still big.